JP5907323B1 - High strength hot dip galvanized steel sheet - Google Patents

Abstract

Provided is a high-strength hot-dip galvanized steel sheet excellent in plating quality and plating appearance of a processed part. It has a specific composition, has a martensite area ratio of 7% or more and less than 25%, a ferrite area ratio of 50% or more, and the internal oxidation amount of the surface iron surface layer is 0.05 g / m 2 or less per side. A cold-rolled steel sheet, an intermetallic compound formed on the cold-rolled steel sheet, containing Al of 0.12 g / m 2 or more and 0.22 g / m 2 or less and containing Fe 2 Al 5 having an average particle size of less than 1.0 μm, and the metal Formed on the intermetallic compound, containing 0.3% ≦ Al% ≦ 0.6% (Al% indicates the Al content (mass%) of the hot-dip galvanized layer), and the surface roughness Ra is Hot dip galvanizing with a glossiness (G value) of 550 or more and 750 or less and a zinc base bottom surface orientation ratio Zn (002) / (004) of 60% or more and 90% or less. A layer.

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet used for an inner plate of an automobile.

  In recent years, in the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets imparted with rust resistance to raw steel sheets, especially hot-dip galvanized steel sheets that can be manufactured at low cost and are excellent in rust resistance have been used. In particular, European and American automakers are thinking of improving rust prevention performance by applying hot-dip galvanized steel sheets that can easily increase the plating thickness, and demand for large steel sheets for automobiles can be expected in the rapidly growing East Asian region. .

  In the case of a steel sheet for automobiles in which good workability is strictly required, durability cannot be maintained unless plating quality such as impact resistance adhesion after press working and corrosion resistance after painting after work is good. Conventionally, it has not been possible to provide hot-dip galvanized steel sheets having sufficient plating quality.

  In particular, a so-called high-strength steel sheet used as a strength member is also required to have high workability and rust prevention of the processed part, and therefore, excellent plating quality is required for the processed part.

  Patent Document 1 discloses a method for producing a hot-dip galvanized steel sheet that is excellent in slidability at the time of press work, which regulates the amount of Al in the plating layer and the amount of Al at the interface between the plating layer and the steel sheet. However, in the technique described in Patent Document 1, the plating quality and plating appearance such as the impact resistance adhesion and the corrosion resistance after coating have not yet been sufficiently considered, and further improvements have been demanded.

JP 2004-315965 A

  This invention is made | formed in view of this situation, Comprising: It aims at providing the high intensity | strength hot-dip galvanized steel plate excellent in the plating quality and plating external appearance of a process part.

As a result of intensive studies, the present inventors do not simply perform hot dip galvanizing treatment as in the prior art, but (1) form an FeAl intermetallic compound with a predetermined property at the plating layer / steel plate interface, (2) In addition to controlling the solidification structure of plating, (3) controlling the texture of the surface, and (4) controlling the state of internal oxidation in the surface layer of the steel sheet, thereby improving the plating quality and plating appearance of the pressed parts. The present inventors have found that an excellent high-strength hot-dip galvanized steel sheet can be provided, and have completed the present invention. The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.06% or more and 0.09% or less, Si: 0.30% or less, Mn: 1.7% or more and 2.3% or less, P: 0.001% or more and 0.00. 020% or less, S: 0.010% or less, Mo: 0.05% or more and 0.30% or less, N: 0.005% or less, Al: 0.01% or more and 0.10% or less, the balance Consists of a composition of Fe and inevitable impurities,
A cold-rolled steel sheet having a structure in which the martensite area ratio is 7% or more and less than 25% and the ferrite area ratio is 50% or more, and the internal oxidation amount of the surface layer portion is 0.05 g / m ² or less per side;
An intermetallic compound containing 0.12 g / m ² or more and 0.22 g / m ² or less of Al formed on the cold-rolled steel sheet and Fe ₂ Al ₅ having an average particle size of less than 1.0 μm;
Formed on the intermetallic compound and containing 0.3% ≦ Al% ≦ 0.6% (Al% indicates the Al content (mass%) of the hot-dip galvanized layer), and the surface roughness Ra is 0.8 μm or more and 1.6 μm or less, and the glossiness (G value) is 550 or more and 750 or less, and the zinc base bottom orientation ratio Zn (002) / (004) defined by the following formula (1) A hot-dip galvanized steel sheet having a hot dip galvanized layer having a thickness of 60% or more and 90% or less.

  In the present invention, the high strength molten steel sheet refers to a steel sheet having a tensile strength (TS) of 590 to 690 MPa.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength hot-dip galvanized steel plate excellent in the plating quality and plating external appearance of a process part is provided.

  The high-strength hot-dip galvanized steel sheet of the present invention has a hot-dip galvanized layer having a specific structure on a cold-rolled steel sheet having a specific structure. In addition, an intermetallic compound having a specific configuration is formed between the cold-rolled steel sheet and the hot-dip galvanized layer. Thereby, the high-strength hot-dip galvanized steel sheet of the present invention is excellent in the plating quality and plating appearance of the processed part. More specifically, the high-strength hot-dip galvanized steel sheet of the present invention is excellent in plating adhesion, post-coating corrosion resistance, and plating appearance during an impact resistance test of a 60 ° bent portion.

  Hereinafter, the present invention will be specifically described.

<Cold rolled steel sheet>
The cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention is C: 0.06% or more and 0.09% or less, Si: 0.30% or less, Mn: 1.7% or more and 2.3% or less. , P: 0.001% to 0.020%, S: 0.010% or less, Mo: 0.05% to 0.30%, N: 0.005% or less, Al: 0.01% or more It contains 0.10% or less, and the balance is composed of Fe and inevitable impurities. Here, first, the reasons for limiting the components of this cold-rolled steel sheet will be described. In addition, “%” of each component in the steel sheet of the present invention indicates “mass%” unless otherwise specified.

[C: 0.06% to 0.09%]
C is one of the important basic components of steel. In the present invention, in particular, the volume fraction of the austenite (γ) phase when heated in the (α (ferrite) + γ (austenite)) region, and thus martensite after transformation. Is an important element to influence the amount of. Mechanical properties such as strength are greatly influenced by the martensite fraction and the hardness of the martensite phase. If the C content is less than 0.06%, the martensite phase is difficult to be generated. On the other hand, if it exceeds 0.09%, the spot weldability deteriorates, so the C content is 0.06% or more and 0.09%. The following.

[Si: 0.30% or less]
Si is an element that improves the workability such as elongation by reducing the amount of solid solution C in the ferrite (α) phase. However, if the Si content exceeds 0.30%, the plating quality is impaired. The upper limit of the content is 0.30%.

[Mn: 1.7% to 2.3%]
In the present invention, Mn is an important element as a basic component because it concentrates in the austenite (γ) phase and promotes martensitic transformation. However, if the Mn content is less than 1.7%, the effect is not obtained. On the other hand, if the Mn content exceeds 2.3%, spot weldability and plating quality are remarkably impaired. Therefore, the Mn content is 1.7% or more and 2.3%. The following.

[P: 0.001% to 0.020%]
P is an element effective in achieving high strength at a low cost. In order to achieve high strength, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.020%, the spot weldability is remarkably impaired, so the upper limit of the P content is 0.020%.

[S: 0.010% or less]
S not only causes hot cracking during hot rolling, but also induces breakage in the nugget of the spot weld, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the S content is suppressed to 0.010% or less.

[Mo: 0.05% or more and 0.30% or less]
Mo is an important element for obtaining a composite structure of ferrite and martensite without impairing the plating quality, and at least the Mo content needs to be 0.05%. However, even if the Mo content exceeds 0.30%, there are few further effects and the manufacturing cost is increased, so the Mo content is 0.30% as the upper limit.

[N: 0.005% or less]
In addition to aging deterioration, N causes an increase in yield point (yield ratio) and yield elongation, so the N content needs to be suppressed to 0.005% or less.

[Al: 0.01% or more and 0.10% or less]
Al is an effective element that fixes N as AlN as a deoxidizer in the steelmaking process and causes aging deterioration, and in order to fully exhibit the effect, the Al content is 0.01% or more. To do. On the other hand, if the Al content exceeds 0.10%, the production cost increases, so the Al content must be suppressed to 0.10% or less.

  The balance of the steel sheet of the present invention consists of Fe and inevitable impurities.

  The cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention described above has a specific structure and physical properties, and details thereof will be described below.

(Martensite area ratio: 7% or more and less than 25%)
In the cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention, when the martensite area ratio is less than 7%, the yield ratio YR increases remarkably. On the other hand, when the area ratio of martensite is 25% or more, the local ductility is lowered and the total elongation EL is lowered. Therefore, in the cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention, the martensite area ratio is 7% or more and less than 25%. The area ratio of this martensite is preferably 7% or more and 22% or less, more preferably 7% or more and 20% or less. The area ratio of martensite mainly controls the C content in steel to 0.06% or more and 0.09% or less, and the heating temperature during annealing (the annealing temperature, which means the highest steel sheet temperature) Can be adjusted to 730 ° C. or higher and 880 ° C. or lower.

(Area ratio of ferrite: 50% or more)
In the cold-rolled steel sheet constituting the high-strength hot-dip galvanized steel sheet of the present invention, when the area ratio of ferrite is less than 50%, the decrease in total elongation EL becomes significant. Therefore, the area ratio of ferrite is 50% or more. The area ratio of this ferrite is preferably 60% or more. The area ratio of ferrite can be adjusted mainly by controlling the C content in the steel to 0.06% or more and 0.09% or less.

  Here, the area ratio of ferrite is the ratio of the area of the ferrite phase in the observation area, and the area ratio of martensite is the ratio of the area of the martensite phase in the observation area. The actual ferrite area ratio and martensite area ratio can be calculated as follows. That is, after the plate thickness direction cross section of the obtained steel plate is polished, it is corroded with 3% nital (alcohol solution containing 3% nitric acid). Then, the vicinity of the ¼ position in the plate thickness direction is observed at a magnification of about 1500 times using an SEM (scanning electron microscope), and the obtained image is analyzed using general image analysis software. The area ratio can be obtained. In the obtained gray scale image, ferrite is gray (underlying structure), and martensite can be identified as a white structure.

(The amount of internal oxidation of the surface layer of the steel sheet on the surface of the cold-rolled steel sheet is 0.05 g / m ² or less per side)
By controlling the state of internal oxidation in the steel sheet surface layer (base metal surface layer), it has excellent plating adhesion and post-coating corrosion resistance during the impact resistance test of the pressed processed parts in high-strength hot-dip galvanized steel sheets Can be. Moreover, the spot weldability can be excellent. In order to ensure good plating adhesion, it is necessary that the amount of internal oxidation of the surface iron surface layer portion on the surface of the cold-rolled steel sheet is 0.05 g / m ² or less per side.

Internal oxidation occurs when oxidizable elements such as Si, Mn, Al, and P contained in the steel sheet are oxidized in a hot rolling process, an annealing process in a CGL (continuous hot dip galvanizing line), or the like. In order to reduce the internal oxidation amount of the surface layer of the steel sheet on the surface of the cold-rolled steel sheet to 0.05 g / m ² or less per side, the coiling temperature at the time of hot rolling should not be increased excessively or in the annealing atmosphere in CGL It is necessary not to raise the dew point excessively. As an appropriate range of dew point, it is preferable not to exceed 0 ° C. in order to avoid deterioration of the roll due to oxidation of the roll surface. Moreover, it is preferable that winding-up temperature does not exceed 700 degreeC for the descalability improvement.

  The surface iron surface layer portion on the surface of the cold-rolled steel sheet refers to a surface layer portion in contact with the hot dip galvanized layer, and can also be said to be a steel sheet surface layer portion immediately under the plated layer after the plating layer is removed. Further, the surface iron surface layer portion in which the internal oxidation amount is defined refers to a range from the interface between the hot dip galvanized layer and the steel plate to 50 μm in the thickness direction of the steel plate.

When the amount of internal oxidation exceeds 0.05 g / m ² per side, grain boundaries in the processed part become brittle, plating adhesion after processing deteriorates, and weldability also deteriorates.

This amount of internal oxidation can be obtained by measuring the amount of oxygen in the steel after the plating layer is removed. The method for removing the plating layer is not particularly limited, but any removal by acid or alkali is possible. However, care should be taken not to remove the base iron and to oxidize the surface after the removal by using an inhibitor (base iron dissolution inhibitor) together. As an example, it is possible to remove the plating layer with a 20% by weight NaOH-10% by weight triethanolamine aqueous solution 195 cc + 35% by weight H ₂ O ₂ aqueous solution 7 cc. In addition, the plating layer can be removed with a dilute HCl solution containing an inhibitor.

  The amount of oxide in steel is measured by, for example, “impulse furnace melting-infrared absorption method”. However, in order to estimate the amount of internal oxidation immediately below the plating layer, it is necessary to subtract the amount of oxygen contained in the base material itself. Therefore, for a sample in which the front and back surface portions of the sample from which the plating layer has been removed are mechanically polished by 100 μm or more. By separately measuring the amount of oxygen in the steel and subtracting it from the amount of oxygen in the sample with the plating layer removed, the amount of increase in oxidation of only the surface layer portion is calculated, and converted into the amount per unit area to obtain a value.

<Intermetallic compound>
Next, the structure of the intermetallic compound formed on the cold-rolled steel sheet described above will be described.

[Al: 0.12 g / m ² or more and 0.22 g / m ² or less]
The high-strength hot-dip galvanized steel sheet of the present invention improves the adhesion at the impact resistance test of the processed part by containing an intermetallic compound at the interface between the plating layer and the steel sheet. This intermetallic compound contains 0.12 g / m ² or more and 0.22 g / m ² or less of Al, and contains Fe ₂ Al ₅ having an average particle size of less than 1.0 μm. Thereby, in the high-strength hot-dip galvanized steel sheet of the present invention, the FeAl intermetallic compound can be formed at the interface between the plating layer and the steel sheet with a fine and dense property.

Here, in order for Al to be less than 0.12 g / m ² in the intermetallic compound, it is necessary to reduce the Al concentration in the hot dip zinc bath of plating, and when this Al concentration is too low, Dross deposits and the appearance deteriorates. On the other hand, in order to make Al more than 0.22 g / m ² in the intermetallic compound, it is necessary to increase the Al concentration in the hot dip zinc bath of plating, and if this Al concentration is too high, the plating layer A large amount of an oxide film of Al is formed on the surface and spot weldability is deteriorated.

(Average particle size of Fe ₂ Al ₅ is less than 1.0 μm)
When the average particle diameter of Fe ₂ Al ₅ is 1.0 μm or more, it is a result of excessive growth of the hard FeAl intermetallic compound, so that the impact resistance characteristics of the high-strength hot-dip galvanized steel sheet of the present invention deteriorate. Therefore, the average particle diameter of Fe ₂ Al ₅ is set to less than 1.0 μm.

  Since the amount of this intermetallic compound produced depends on the activity of Al in the molten zinc bath, it can be increased mainly by increasing the Al concentration. However, if there is too much Al, the impact resistance characteristics deteriorate as described above, so it is necessary to control to an appropriate amount.

The average particle diameter of this Fe ₂ Al ₅ can be adjusted by controlling the Al concentration in the plating bath.

  In addition, although said average particle diameter is not specifically limited, It can measure with the measuring method set to the predetermined magnification using a scanning electron microscope (SEM).

<Hot galvanized layer>
Next, the structure of the hot dip galvanized layer formed on the surface of the cold rolled steel sheet and formed on the intermetallic compound described above will be described.

[Zn and 0.3% ≦ Al% ≦ 0.6%]
The hot-dip galvanized layer constituting the high-strength hot-dip galvanized steel sheet of the present invention contains Zn and 0.3% ≦ Al% ≦ 0.6%. Here, Al% indicates the Al content (mass%) of the hot-dip galvanized layer. In order to make Al less than 0.3%, it is necessary to lower the Al concentration in the hot dip zinc bath of plating. If this Al concentration is too low, Fe elution will occur and dross will precipitate. As a result, the appearance deteriorates. If the Al content exceeds 0.6%, a large amount of an oxide film of Al is formed on the surface of the plating layer, and the spot weldability deteriorates. The hot-dip galvanized layer can also contain Pb, Sb, Mg, Ni, Mn, Si, Ti, Cr, Sr, and Ca.

(Surface roughness Ra of hot dip galvanized layer surface: 0.8 μm or more and 1.6 μm or less)
When the surface roughness Ra of the hot dip galvanized layer surface is less than 0.8 μm, oil is not retained during pressing, and press formability during processing is poor. On the other hand, if Ra is more than 1.6 μm, post-paint clarity and adhesion are poor. Therefore, Ra is set to 0.8 μm or more and 1.6 μm or less.

  In the adjustment of Ra, an appropriate amount is secured by using a dull roll subjected to high roughness processing in the skin pass process. In the dull adjustment method, a shot dull, EDT (Electron discharged texturing), EBT (Electron beam texturing), or a scratch dull processed dull roll is used. In addition, although said Ra is not specifically limited, Based on JISB0601 (2001), it can measure with a roughness meter.

(Glossiness (G value) of hot dip galvanized layer surface: 550 or more and 750 or less) and (Zinc base bottom surface orientation ratio: Zn (002) / (004) is 60% or more and 90% or less)
In the high-strength hot-dip galvanized steel sheet of the present invention, in order to control the solidification structure of the plating, first, the glossiness (G value) is set to 550 or more and 750 or less, and the zinc base bottom orientation defined by the following formula (1) The rate Zn (002) / (004) is 60% or more and 90% or less. By setting the zinc base basal plane orientation ratio Zn (002) / (004) to 60% or more and 90% or less, the zinc of hcp (hexagonal close-packed structure) is solidified when solidified. Can be made easier.

  Regarding the solidification structure of the plating, mainly, when the Al concentration in the bath reaches a predetermined value, an Fe-Al intermetallic compound is formed at the interface between the plating phase and the steel sheet, and the solidification structure of zinc is developed soundly. If the Al concentration is too high, the solidified structure is formed in a dendritic shape, so that the surface is uneven and the glossiness (G value) is reduced to less than 550. Conversely, when the Al concentration is low, the formation of the Fe—Al intermetallic compound at the interface is suppressed, and at the same time, the Fe—Zn alloy layer develops. Thereby, in order to increase the base point of the Zn solidification nucleus, the solidification structure is refined and smoothed, whereby the glossiness (G value) increases more than necessary and exceeds 750. When the glossiness (G value) is less than 550, it corresponds to adding excess Al to the zinc bath, and the spot weldability is deteriorated. Further, when the glossiness (G value) exceeds 750, it corresponds to a case where the amount of Al added to the zinc bath is small, and Fe is eluted to cause surface defects due to dross.

  The glossiness (G value) is not particularly limited, but can be measured with a glossometer based on JIS Z 8741 (1997). The desired surface texture can be ensured by controlling the texture with a skin pass or the like.

  In addition, when the zinc base bottom surface orientation ratio Zn (002) / (004) is less than 60%, since the crystal size when zinc solidifies immediately after plating becomes small when the orientation of the zinc crystal is relatively random, It is too smooth and oil is not retained on the steel sheet during pressing, resulting in poor formability. If the orientation ratio of the basal plane of the zinc base exceeds 90%, the orientation of the basal plane of the Zn crystal is too high and the crystal grains are likely to grow. As a result, the dendritic arm develops. Deteriorate. The zinc base bottom surface orientation ratio Zn (002) / (004) is not particularly limited, but can be obtained by measuring the X-ray diffraction intensity.

  Zn has an hcp structure and is usually easily oriented on the basal plane, but the measurement of the zinc basal plane orientation ratio represented by the formula (1) shows how much the crystals are randomly oriented. The gloss, crystal size, and surface roughness are affected by the degree of orientation of the solidified structure, so accurate control of the zinc base orientation ratio is not only the surface properties of high-strength hot-dip galvanized steel sheets, but also press workability. It is extremely important when controlling. As described above, the orientation is adjusted by securing an appropriate amount of the Fe—Al alloy layer at the plating phase / steel plate interface and suppressing the formation of the Fe—Zn alloy layer serving as a precipitation nucleus of the zinc solidified structure.

(Tensile strength (TS): 590 MPa to 690 MPa)
In the high-strength hot-dip galvanized steel sheet of the present invention, the tensile strength (TS) is set to 590 MPa or more and 690 MPa or less in order to mainly secure the strength of the inner plate. Such a high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 590 MPa or more and 690 MPa or less can be obtained by passing the CGL under appropriate annealing conditions for the material of the steel sheet component in the present invention. The annealing temperature is preferably about 800 to 850 ° C. because annealing in a two-phase region is necessary.

[Plating adhesion (impact adhesion) and impact resistance after coating during impact resistance test of processed parts]
In order to simulate the processing by the impact at the time of the collision of the automobile by the machining part such as the side member of the automobile, a portion bent by 90 ° is targeted. This portion is examined for impact resistance with a so-called DuPont tester, and evaluated by peeling the tape. Corrosion resistance after coating is evaluated by the swollen corrosion width in the SST test after chemical conversion treatment and electrodeposition coating are performed on the bent portion, cross-cut. If the amount of the Fe—Al intermetallic compound at the interface can be ensured more than necessary as described above, good plating adhesion (impact adhesion) can be ensured.

<Method for producing high-strength hot-dip galvanized steel sheet>
Then, the manufacturing method of a high intensity | strength hot-dip galvanized steel plate is demonstrated. For example, a high-strength hot-dip galvanized steel sheet can be produced by the following method. First, steel having the above component composition is made into a slab by continuous casting, the slab is heated, and scale removal and rough rolling are performed. Next, after cooling, finish rolling, cooling, winding, pickling, and cold rolling are performed. Next, the steel sheet is annealed and hot-dip galvanized in a continuous hot-dip galvanizing facility.

  The heating time, heating temperature, rough rolling conditions, cooling conditions, finish rolling conditions, winding conditions and the like when heating the slab can be appropriately set based on common technical knowledge. However, in order to adjust the amount of internal oxidation at the surface layer portion in the present invention to the above range, it is preferable to adjust the conditions for finish rolling (hot rolling) and the coiling temperature.

  In addition, the annealing conditions of the steel sheet affect the yield stress of the high-strength hot-dip galvanized steel sheet. In the present invention, in order to set the tensile strength within the above range, the heating temperature during annealing (the annealing temperature, which means the highest steel sheet temperature) is set to 730 ° C. or higher and 880 ° C. or lower, preferably 800 It sets to 850 degreeC or more.

  The annealing atmosphere may be adjusted as appropriate, but in the present invention, the dew point is preferably adjusted to 0 ° C. or lower. Exceeding 0 ° C. is not preferable because the surface of the furnace body tends to become brittle.

The hydrogen concentration in the annealing atmosphere is preferably 1 vol% or more and 50 vol% or less. If the hydrogen concentration is 1 vol% or more, it is preferable because the surface of the steel sheet is activated, and if the hydrogen concentration exceeds 50 vol%, it is not preferable because it is economically disadvantageous. Normally containing N ₂ is other than hydrogen. Inevitable components include CO ₂ , CO, O _{2 and the} like.

  In the present invention, in order to control the Al content of the hot dip galvanized layer and allow an intermetallic compound to exist between the steel sheet and the hot dip galvanized layer, it is necessary to adjust the conditions of the hot dip galvanizing treatment. Moreover, in order to make the surface state (surface roughness Ra, glossiness (G value), zinc base bottom orientation ratio) of a hot dip galvanized layer into a desired state, it is necessary to adjust the conditions of the hot dip galvanizing treatment. . Hereinafter, conditions for the hot dip galvanizing process will be described.

  The infiltration plate temperature, which is the temperature of the steel plate when the annealed steel plate enters the plating bath, is not particularly limited, but is preferably a plating bath temperature (bath temperature) of −20 ° C. or higher and a bath temperature of + 20 ° C. or lower. If the infiltration plate temperature is in the above range, the change in bath temperature is small, and it is easy to perform desired hot dip galvanization continuously. The Al content in the hot-dip galvanized layer and the Al content in the intermetallic compound tend to decrease by increasing the bath temperature. Moreover, the glossiness of the surface of the hot dip galvanized layer tends to increase as the bath temperature is increased.

  The composition of the plating bath into which the steel sheet after annealing penetrates may be any composition that contains Al in addition to Zn, and may contain other components as necessary. The concentration of Al in the plating bath is not particularly limited, but is preferably 0.16% by mass or more and 0.25% by mass or less. An Al concentration in the above range is preferable because an Fe—Al alloy phase is formed and the Fe—Zn alloy phase is suppressed. The glossiness can be adjusted by the Al concentration in the plating bath. When the Al concentration in the plating bath decreases, Fe-Zn crystals, not Fe-Al, are formed slightly at the interface, and these become Zn solidification nucleation sites, so that a large number of zinc crystals are generated, and the zinc crystal orientation is random. As a result, the orientation rate tends to decrease. As a result, as the Al concentration is lower, dendritic Zn crystal growth is suppressed, and surface unevenness is reduced and smoothed, so that the glossiness increases. A more preferable Al concentration is 0.19% by mass or more and 0.22% by mass or less. Since the Al concentration also affects the Al content in the hot-dip galvanized layer and the Al content in the intermetallic compound, it is preferable to determine the Al concentration in consideration of these contents.

  The temperature of the plating bath (bath temperature) is not particularly limited, but is preferably 430 ° C. or higher and 470 ° C. or lower. A bath temperature of 430 ° C. or higher is preferred for the reason that the zinc bath dissolves stably without solidification, and a bath temperature of 470 ° C. or lower is preferred for the reason that Fe elution is small and dross defects are reduced. A more preferable range of the bath temperature is 450 ° C. or higher and 465 ° C. or lower.

  Although the immersion time at the time of immersing a steel plate in a plating bath is not specifically limited, It is preferable that they are 0.1 second or more and 5 seconds or less. When the immersion time is in the above range, a desired hot dip galvanized layer is easily formed on the surface of the steel plate.

Immediately after lifting the steel plate from the plating bath, the amount of plating adhesion is adjusted by gas jet wiping or the like. In the present invention, the plating adhesion amount is not particularly limited, but is preferably in the range of 20 g / m ² or more and 120 g / m ² or less. If it is less than 20 g / m ² , it may be difficult to ensure corrosion resistance. On the other hand, if it exceeds 120 g / m ² , the plating peel resistance may deteriorate.

  After adjusting the plating adhesion amount as described above, temper rolling (SK treatment) is performed. The type of roll used for the SK treatment is not particularly limited, and an Electro-Discharge Texture roll (EDT roll), an Electron Beam Texture roll (EBT roll), a shotdal roll, a topochrome roll, and the like can be used.

  The rolling reduction rate during SK treatment (SK rolling reduction rate (%)) is not particularly limited, but is preferably 0.7 to 0.9%. If the SK rolling reduction is in the above range, the surface roughness can be easily adjusted to the above preferable range. Further, if it is outside the above range, the press workability may be deteriorated without a dull eye that holds the lubricating oil, and the yield strength may also be lowered.

  The cooling rate after the steel sheet is pulled up from the plating bath is preferably −5 ° C./second or more and −30 ° C./second or less.

  As described above, the high-strength hot-dip galvanized steel sheet of the present invention has been described. Hereinafter, the use of the high-strength hot-dip galvanized steel sheet of the present invention will be described.

  The high-strength hot-dip galvanized steel sheet of the present invention is preferably used for applications in which a coating film is formed on the surface of the hot-dip galvanized layer because it is excellent in post-coating corrosion resistance after press working. In addition, the high-strength hot-dip galvanized steel sheet of the present invention is excellent in plating adhesion even when applied to applications requiring strict workability, and does not significantly reduce corrosion resistance and mechanical properties. Examples of applications in which strict processability is required and a coating film is formed include automotive steel plates such as automobile outer plates and inner plates. Although the formation method of a coating film is not specifically limited, After performing a chemical conversion treatment to the surface of a hot-dip galvanized layer and forming a chemical conversion film, it is preferable to form a coating film on this chemical conversion film.

  Either a coating type or a reaction type can be used as the chemical conversion treatment liquid. Moreover, the component contained in a chemical conversion liquid is not specifically limited, either a chromate processing liquid may be used and a chromium free chemical conversion liquid may be used. Moreover, the chemical conversion film may be a single layer or a multilayer.

  The coating method for forming the coating film is not particularly limited, and examples of the coating method include electrodeposition coating, roll coater coating, curtain flow coating, and spray coating. In order to dry the paint, means such as hot air drying, infrared heating, induction heating and the like can be used.

  Hereinafter, the present invention will be specifically described based on examples. The present invention is not limited to the following examples.

The steel sheet as shown in Table 1 was removed by pickling the hot-rolled steel sheet produced by winding at a coiling temperature of 650 ° C. or less, and cold rolled at a cold rolling reduction of 50%. A cold-rolled material having a thickness of 1.2 mm or 2.3 mm was produced. Thereafter, the surface was degreased and subjected to annealing and hot dip galvanizing treatment under the conditions shown in Table 2. The line speed (LS) was 60 mpm or 100 mpm. The bath temperature and the Al concentration in the bath were appropriately changed. After adjusting the plating adhesion amount, temper rolling (SK treatment) was performed, and the roll used for the SK treatment was changed appropriately by using an EDT processing roll. The amount of adhesion was 55 g / m ² per side. The results are shown in Table 2.

  Regarding the obtained high-strength hot-dip galvanized steel sheet, first, as appearance (plating appearance), it was visually judged as good (◯) when there was no appearance defect such as uneven plating, and when it was present, it was judged as defective (×).

  Moreover, based on JISZ8741 (1997), 60 degree specular glossiness (G value) was measured with the glossiness meter.

  Using the θ-2θ scan method with an X-ray diffractometer, the Zn crystal orientation on the (002) plane and the (004) plane on the (004) plane was measured with X-rays, and the zinc base bottom orientation ratio on the surface of the hot dip galvanized layer Zn (002) / (004) was measured.

  Based on JIS B0601 (2001), the surface roughness Ra of the hot dip galvanized layer surface was measured with a roughness meter.

  The Al content of the hot dip galvanized layer was peeled off with dilute hydrochloric acid containing an inhibitor, and quantified by ICP emission spectrometry.

As the composition of the intermetallic compound, the FeAl-enriched layer amount (total mass of the Fe ₂ Al ₅ alloy layer) was determined by peeling the galvanized layer with fuming nitric acid and quantifying the FeAl-enriched layer amount as Al by ICP emission analysis.

The average particle diameter of Fe ₂ Al ₅ constituting the intermetallic compound was measured by observing it at a magnification of 5000 using a scanning electron microscope (SEM).

Whether or not the composition of the intermetallic compound was Fe ₂ Al ₅ was determined by thin film X-ray diffraction.

  The amount of internal oxidation was obtained by measuring the amount of oxygen in the steel after removing the plating layer. The amount of oxide in steel was measured by “impulse furnace melting-infrared absorption method”. In order to estimate the amount of internal oxidation immediately below the plating layer, it is necessary to subtract the amount of oxygen contained in the base material itself. Therefore, the steel of the sample obtained by mechanically polishing the front and back surface portions of the sample from which the plating layer has been removed 100 μm or more. Separately measure the amount of oxygen in the medium, and subtract from the amount of oxygen in the sample with the plating layer removed to calculate the amount of increase in oxidation only on the surface layer, and convert it to the amount per unit area to obtain the value of internal oxidation It was.

  For measurement of the martensite area ratio and ferrite area ratio, first, the cross section in the plate thickness direction of the obtained steel plate was polished, and then corroded with 3% nital (alcohol solution containing 3% nitric acid). Then, the vicinity of the ¼ position in the plate thickness direction is observed at a magnification of about 1500 times using an SEM (scanning electron microscope), and the obtained image is analyzed using image analysis software, and the area ratio of each phase. Asked. In the obtained gray scale image, ferrite was gray (underlying structure), and martensite was identified as a white structure.

  The impact-resistant adhesion of the processed part is 1843 g on the outer convex part of the bent part of a part 80 mm long × 30 mm wide bent at 60 ° with a bending R = 1.5 mm at a position 40 mm in the longitudinal direction. The impact resistance test was performed by dropping a punch having a hitch diameter of 5/8 inch from a height of 1 m, and peeling with a Nichiban tape was observed. Those with peeling were marked with x, and those without peeling were marked with ◯.

○: Good adhesion ×: Poor adhesion

  In addition, chemical treatment, electrodeposition coating, intermediate coating, and top coating were comprehensively applied to the parts subjected to the same processing treatment, and the corrosion resistance after coating was investigated. A salt spray test based on JIS Z 2371 (2000) was conducted for 10 days to evaluate the presence or absence of significant swelling on the outside of the bent portion.

Good (○): No blistering failure (×): With blistering

  A JIS No. 5 tensile test piece is taken from the sample in the direction of 90 ° with respect to the rolling direction, and subjected to a tensile test at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241 to obtain a tensile strength (TS (MPa)). Was measured.

As is apparent from Table 2, the high-strength hot-dip galvanized steel sheet of the present invention had very good characteristics despite the press work, no plating peeling, and excellent impact resistance adhesion. Moreover, the corrosion resistance after coating was also good. The plating appearance was also good.

Claims (1)

By mass%, C: 0.06% to 0.09%, Si: 0.30% or less, Mn: 1.7% to 2.3%, P: 0.001% to 0.020% , S: 0.010% or less, Mo: 0.05% or more and 0.30% or less, N: 0.005% or less, Al: 0.01% or more and 0.10% or less, with the balance being Fe and Consisting of the composition of inevitable impurities,
It has a structure in which the martensite area ratio is 7% or more and less than 25%, and the ferrite area ratio is 50% or more.
A cold-rolled steel sheet in which the internal oxidation amount of the surface iron surface layer part is 0.05 g / m ² or less per side;
An intermetallic compound formed on the cold-rolled steel sheet, containing 0.12 g / m ² or more and 0.22 g / m ² or less of Al and Fe ₂ Al ₅ having an average particle size of less than 1.0 μm;
Formed on the intermetallic compound;
0.3% ≦ Al% ≦ 0.6% (Al% indicates the Al content (mass%) of the hot-dip galvanized layer),
The surface roughness Ra is 0.8 μm or more and 1.6 μm or less,
Glossiness (G value) is 550 or more and 750 or less,
A hot-dip galvanized layer having a zinc base bottom surface orientation ratio Zn (002) / (004) defined by the following formula (1) of 60% or more and 90% or less;
High strength hot dip galvanized steel sheet.